Olivine-structured, lithium-containing phosphate compounds (hereinafter, also simply referred to as olivine-structured active materials) such as LiFePO4 and LiMnPO4 are known as positive electrode active materials for lithium ion secondary batteries, being excellent in thermal stability and low in cost. However, an olivine-structured active material is higher in electric resistance, compared to a hexagonal, layer-structured metal composite oxide such as a lithium cobalt composite oxide that has been widely used from the past. Therefore, there was the disadvantage of its slowing down intercalation and release reactions of lithium ions, and causing low discharge capacity at times of high rates. Also, when an olivine-structured active material deteriorates due to repeated charge and discharge of the battery, metal elements such as Fe and Mn are likely to be eluted into the non-aqueous electrolyte. Therefore, there was also the disadvantage of its causing deposition of the eluted Fe and Mn onto the negative electrode surface, resulting in capacity drops and internal short circuits in the battery and thus causing deterioration in cycle characteristics of the battery. Such elution of metal elements is remarkable, particularly in instances where charge and discharge are repeated under high temperatures.
With the aim to suppress elution of Fe from an olivine-structured active material, PTL 1 discloses forming a coating layer of a lithium compound on the surface of olivine-structured active material particles. Also, PTL 2 discloses forming a conductive carbon layer on the surface of olivine-structured active material particles which include iron or manganese, or between such particles. By forming a conductive carbon layer, PTL 2 aims to suppress elution of Fe and Mn from an olivine-structured positive electrode active material, and also, to make the olivine-structured active material, conductive.
In addition, with the aim to provide lithium ion secondary batteries with higher capacity and longer life, PTL 3 discloses a positive electrode active material for lithium ion secondary batteries, the active material including a composite oxide represented by the following formula:Lix+aMn2−a−bDbQcO4+d where: D is one or more selected from Ni, Fe, Co, Cu, and Cr; Q is one or more selected from C, N, S, P, Si, F, Cl, I and Br; and x, a, b, c, and d have ranges of 0≦x≦1.1, 0≦a≦0.5, 0.05≦b≦1.0, 0.000001≦c<0.05, and 0≦d≦0.1, respectively. PTL 3 also discloses making the concentration of the element Q higher on the surface layer of the particles than in the inside thereof.